Modular Link Layer Functions of a Generic Protocol Stack for Future Wireless Networks

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MODULAR LINK LAYER FUNCTIONS OF A GENERIC PROTOCOL STACK FOR FUTURE WIRELESS NETWORKS

Lars Berlemann, Arnaud Cassaigne, Ralf Pabst, Bernhard Walke (ComNets, RWTH Aachen University, Aachen, Germany) ber|adc|pab|[email protected]

ABSTRACT mode architectures can already be found in existing systems, like IEEE 802.16 [3]. Multi-mode capable wireless networks are a key issue in Software Defined Radios (SDRs), [4] and [6], are a future wireless communication. This paper introduces promising approach towards these multi-mode devices. therefore the realization and application of a generic The recent technological progress enables an extension of protocol stack as common part of a multi-mode capable the key issues in research of SDR from the signal communication protocol software. This can be regarded as processing of the physical layer on the complete an extension of the field of software defined radios with communication chain used for wireless communication as its origin in the physical layer on the upper layers of the enabling technology for cognitive radios [4], [5] and [7]. protocol stack. The generic protocol stack compromises Therefore, this paper extends the focus of SDRs on the common functionality and behavior of the communication communication protocol software in considering the Data protocols that is extended through specific parts of a Link Layer (DLL) and network layer corresponding to the dedicated radio access network technology. In a bottom- ISO/OSI reference model [8]. This work supplements the up approach, this paper considers fundamental protocol research of the integrated project E2R [9] dealing with functions realized as parameterizable modules. These end-to-end reconfigurability. protocol functions originally correspond to the data link After introducing the idea of a generic protocol stack layer of the ISO/OSI reference model. The system specific in Section 2, its application for protocol convergence in aspects of the protocol software are realized through the context of a multi-mode capable protocol architecture adequate parameterization of the modules’ behavior. is outlined. The realization of a generic protocol stack on Further specific functionality and behavior can be added basis of fundamental protocol functions that can be to the generic protocol stack through inheritance or parameterized is summarized in Section 3. The insertion of system specific modules. Thus, the generic composition of specific protocol layers of adequately protocol stack enables an efficient as well as flexible parameterized modules is shown in Section 4 followed by realization of the protocol software as part of a future an evaluation of the segmentation/reassembly module at communication network of multiple radio access the example of channel utilization with and without technologies. concatenation in Section 5. This paper ends with a summary and outline of future work as part of the conclusion. 1. INTRODUCTION 2. THE GENERIC PROTOCOL STACK The radio access of future ubiquitous communication networks will be released from the constrains of cellular The rationale for approaching a generic protocol stack is wireless networks, as for instance Universal Mobile that all communication protocols share much functional Telecommunication System (UMTS), or Wireless Local commonality, which can be exploited to build an efficient Area Networks (WLANs). Wireless mobile broadband multi-mode capable wireless system. The aim is to gather systems, providing a patchy coverage in densely these common parts in a single generic stack and populated urban areas, will play an important role. For specialize this generic part following particular details on such a fixed and planned relay based radio requirements of the targeted RAT, as depicted in Fig. 1. network see [1] and [2]. The addressed future wireless The targeted advantages of this concept are: runtime network will have to combine several Radio Access reconfigurability and maintainability, code/resource Technologies (RATs): Consequently, multi-mode capable sharing and protocol development acceleration through terminals as well as base stations are required to enable reusability. the seamless interworking between these RATs. Multi- The initial step towards a generic stack is a detailed, layer-by-layer analysis of communication protocols to architecture of the ISO/OSI reference model, it is rather multi-mode difficult to identify similarities and attribute these to (classic) single protocol stack protocol stack specific layers. Therefore, this paper deepens the level of examination and considers fundamental protocol functions same property system specific cross stack protocol stack management as one basis for a generic protocol stack, contrary to [10] specific functionality common functionality and [11] where complete protocols are analyzed for specific parts generic genericity. The identified protocol functions correspond protocol stack mainly to the DLL as specified in the ISO/OSI reference RAT 1 RAT x model. Nevertheless they can be found in multiple layers different RAT 2 protocol protocol data of today’s protocol stacks as for instance the function of functions framework structures segmentation and reassembly or an Automatic Repeated Fig. 1: UML diagram of the generic protocol stack in the Request (ARQ) protocol for error correction which is context of multi-mode capable networks. located in the Radio Link Control (RLC) as well as in the transport layer, namely in the Transmission Control identify their similarities. Their elaborated realization of Protocol (TCP). the generic parts is crucial for the success of the proposed concept in the face of a tradeoff between genericity, i.e. 2.2. Enabling Protocol Convergence of Future general usability, and implementation effort. As depicted Wireless Networks in Fig. 1, the generic protocol stack comprises fundamental protocol functions, data structures and an The generic protocol stack enables the protocol architectural framework, which form, together with RAT convergence of future wireless networks. The specific parts, a system specific protocol stack. An convergence of such multi-mode protocol stacks has two efficient multi-mode capable stack is realized in adding dimensions: On the one hand the convergence between cross stack management related functions. The cross stack two adjacent layers, in the following referred to as vertical management of the generic protocol stack for enabling convergence, and on the other hand the convergence protocol reconfigurability is introduced in detail in [14]. between layers located in the different modes of the From the software engineering perspective, there are protocol stack which have the same functions: In the in general two possibilities for approaching the generic following referred to as horizontal convergence. The protocol stack: (1.) Parameterizable functional modules generic protocol stack, as introduced above, enables both and/or (2.) inheritance, depending on the abstraction level the horizontal as well as the vertical protocol of the identified protocol commonalities. As introduced convergence. above this paper focuses more on the modular approach Fig. 2 depicts the transition between two protocol while the inheritance-based approach is in considered stacks of different air interface modes of a future wireless in [10] and [11]. Additionally, [12] takes up the idea of a network. The different Physical Layer (PHY) options of generic protocol stack in focusing on a generic link layer IEEE 802.16 could serve as an example, see [3]. In for the cooperation of different access networks at the present, these PHY options are not envisaged to co-exist level of the data link layer. However, not only the link in terminal or access equipment, although they share a layer protocols have to be considered in a multi-mode common Medium Access Control (MAC) protocol with capable network but also higher layer functions as for very little option-specific extensions to the MAC. The instance the control and management of the radio protocol stack is separated into the user and control plane resources as well as mobility. (u- and c-plane) on the one hand and the management plane (m-plane) on the other hand. The split between 2.1. Identifying Commonalities common and specific parts of the protocol is here exemplary depicted for the MAC layer, as explained The evolution of the digital cellular mobile radio networks above. This split may be also necessary in the RLC or originated in the Global System for Mobile higher layers, depending on the targeted protocol Communication (GSM) toward systems of the third architecture and functional flexibility of the common part. generation, as for example UMTS, has shown that in their A cross stack management logically connects the protocol standardization it is fallen back on well-proven functions stacks of the different modes on the m-plane. and mechanisms which are adopted to the specific A seamless interworking and optimized transition requirements of their application. The approach towards between mode 1 and mode 2 has certain requirements to an efficient multi-mode protocol stack, introduced in this the cross stack management: The protocol data, as for paper, is based on these protocol commonalities. instance the protocol status information of the existing As the architecture of modern communication connections has to be transferred between the two modes. protocols cannot be forced into the classical layered

Fig. 2: A multi-mode capable protocol architecture under consideration of an optimized protocol convergence. Protocol information is transferred between two modes, here exemplary depicted on the level of the MAC layer as it is the case in IEEE 802.16 [3], with the help of a of a cross stack management. This horizontal convergence is performed by a mode is done based on generic service primitives and generic convergence protocol. Therefore, protocol functions of Protocol Data Units (PDUs) which are also considered as the c-plane, common to the different modes, are necessary being a part of the generic stack, see again Fig. 1. to access u-plane status information. These common A unique manager as well as interfaces for the functions rely on a well-defined interface towards the Service Access Points (SAP) to the adjacent layers mode specific part of the layer. For further details on the complete the fully functional protocol layer as depicted in proposed protocol architecture and corresponding Fig. 3. In detail, the mentioned components have the infrastructure see [13]. following tasks: The cut between the common and specific part has a • Functional Module (generic or RAT specific): step to indicate that there is the need for (1.) a mode- Realizes a certain fundamental functionality as black specific interface to the PHY as said interface may box. In case of a generic module, a list of parameters inherently not be common, and (2.) a MAC layer internal for characterizing the functionality is given and the interface for the communication between the common and underlying functionality is hidden. The specific part to enable for instance the vertical protocol comprehensiveness of the fulfilled function is limited convergence. to fit straightforward into a single module. The generic protocol stack is the realization of the • Manager: Composes and administers the layer during common part, illustrated in Fig. 3, and implements its runtime. This implies the composition, rearrangement, common functions based on modules. These modules are parameterization and data questioning of the concreted through parameterization adequate to the functional modules. Additionally, the manager targeted specific mode. administers the layer internal communication, as for instance the connection of the layer’s modules through 3. MODULAR APPROACH TOWARDS THE generic service primitives. The manager is the layer’s REALIZATION OF A GENERIC PROTOCOL counterpart of the protocol reconFiguration manager STACK as introduced above in Fig. 2. The manager realizes the protocol convergence of the layer. The generic protocol stack is the realization of the • Generic interface: Translates the generic service common parts, as illustrated in Fig. 1, and implements its primitives with specific protocol information as common functions based on modules. These common payload to system specific ones and enables thus the protocol functions get their system specific behavior based vertical as well as horizontal integration of the system on parameterization. Once specified, these modules can be specific parts of the layer. repeatedly used with a different set of parameters corresponding to the specific communication system. The • Service Access Point (SAP): Here, services of the modules of generic protocol functions - form together layer are performed for the adjacent layers. The layer with system specific modules - a complete protocol layer, may communicate via generic primitives without a as depicted in Fig. 3. The communication inside said layer translation interface to an adjacent layer if said layer has the same modular composition. The interface is to RRC/PDCP/BMC (radio bearers) SAP service access point

interface interface system- SAP acknow- interface functional specifc multiplexer ledged interface module mode to RRC transparent unacknow- generic mode ledged segmentation unit control FunctionalFunctional mode manager functional manager moduleModulModul segmentation unit ARQ unit

functional PDU factory PDU factory layer or module multiplexer sublayer data interface interface service access point SAP Fig. 3: Composition of a protocol specific layer or sublayer to MAC (logical channels) based on generic and system-specific functional modules. Fig. 4: UMTS RLC layer based on the functional modules of the generic protocol stack.

needed if it is demanded that the layer appears as a to higher layer classic layer ﬁtting into an ordinary protocol stack. interface • PDU factory (as functional module, later depicted in Fig. 4-6): Composes layer speciﬁc protocol frames and multiplexer places them as payload in generic PDUs. The introduced modular concept enables the ARQ ARQ ARQ ARQ simulation and performance evaluation of the protocol IP address manager multiplexer multiplexer software on several levels of abstraction: A single (sub-) TCP port UDP port layer as well as a complete protocol stack can be TCP PDU factory UDP PDU factory composed out of the introduced functional modules. multiplexer

segmentation unit 3.1. Generic Protocol Functions of the Data Link Layer IP PDU factory interface This section introduces generic protocol functions of the data link layer, realized as parameterizable modules, as to data link layer Fig. 5: TCP, IP and UDP layer based on the functional initial step in the implementation of a protocol software modules of the generic protocol stack. The TCP layer (gray for a multi-mode capable future wireless system as dash-dotted line) is considered in [14]. introduced in Section 2. The following link layer functions may be part of the to LLC SAP generic protocol stack

• Error handling with the help of ARQ protocols: Send- SAP interface mobility unit and-Wait ARQ, Go-back-N or Selective-Reject ARQ interface multiplexer or Forward Error Correction (FEC) to STA mobility/data MGMT layer • Flow control* segmentation unit • Segmentation, concatenation and padding of PDUs* manager ARQ unit • Discarding of several times received segments* • Reordering of PDUs* PDU factory • Multiplexing/De-Multiplexing of the data flow, as for interface instance the mapping of different channels* SAP • Dynamic scheduling to PHY Fig. 6: 802.11 MAC layer based on the functional modules • Ciphering of the generic protocol stack. • Header compression PDU exceeds a certain predefined limit, given for instance The functions marked with a star* are considered in through the capacity of a physical channel. The PDU is the following section while further functions are separated into different segments that are transmitted considered in [14]. Segmentation is used if the length of a separately. Contrary the concatenation, where several upper-layer SDUs are combined in the same data packet Software and Environment for Protocol Simulation to reach a predefined PDU length. (MOSEPS) that provides basic traffic generators, a model of an erroneous channel and statistical evaluation 3.2. Parameterization of Functional Modules methods. Once specified, these modules can be repeatedly In this context parameterization implies not only specific used with a different set of parameters corresponding to values, as for instance the datagram size of a segmentation the specific communication system. This section module, but also a conFiguration of behavior and introduces the modular approach to protocol functions characteristics of a module, as for example the concretion with a focus on the segmentation/reassembly module at of an ARQ module as a Go-back-N ARQ protocol with the example of the approach to a realization of an UMTS specified window sizes for transmission and reception. RLC as depicted in Fig. 4. This implies as well a conFiguration of the modules’ interface to the outside. The parameterization of 5.1. Analytical Evaluation of functional modules may imply (1.) a specification of Segmentation/Reassembly Module certain variables, (2.) the switching on/off of certain In general, segmentation is needed in all cases where functionality/behavior and (3.) an extension of the higher layer PDUs, referred to as Service Data Units module’s interface to the outside. (SDUs), need to be separated into multiple PDUs to be Taking the example of the segmentation/reassembly further handled by lower layers. This restriction module, the parameterization may imply among other concerning the size of a PDU may result for instance from things: reasonable limitations of a PDU transmitted with error • Use of concatenation correction in an ARQ protocol or may be motivated through the capacity of a transport channel offered by the

• Use of padding, i.e. ﬁlling up of the PDU to reach a physical layer, using the notation of UMTS. certain size In case of multiple users sharing a common channel, • Transmitter or/and receiver role the segmentation can increase the channels efﬁciency in • Buffer size for SDUs concatenated in a single PDU having a multiplexing gain. The above-introduced • Size of PDU after handling concatenation increases thereby the channel utilization as it is outlined in Fig. 7. The channel utilization as quotient

• Behavior in case of error, i.e. interworking with ARQ of user payload and available channel capacity is given module through

payload l payload 4. COMPOSITION OF SYSTEM SPECIfiC LAYERS = (1) l channel capacity payload i packet size packet size As introduced above, the link layer functions are not in the case of no concatenation. The packet size of the limited in their appearance to the DLL. To illustrate the physical channel is assumed to be fixed and the packet applicability of the modular approach, a composition of length of the user data l determines if the segmented three exemplary protocol layers, all differently localized payload higher layer SDU(s) fit(s) into a single PDU. We assume in a protocol stack corresponding to the ISO/OSI that an additional physical channel is established if the reference model, is introduced in the following: (1.) A user data requires it. Thus, additional channel capacity is UMTS RLC layer in Fig. 4, (2.) a TCP, IP and UDP layer provided and the available capacity is increased. In case in Fig. 5 and (3.) a IEEE 802.11 MAC layer in Fig. 6. We of no concatenation the fixed-sized packet is transmitted limit the consideration of Fig. 5 to the TCP layer, marked partly empty over the physical channel. Consequently, the through the gray dash-dotted rectangle. The medium channels’ overall utilization, i.e. the effectively used access of the Distributed Coordination Function (DCF) of capacity compared to the amount of provided capacity, is 802.11 may be regarded as a Send-and-Wait ARQ, simply decreased as illustrated by the solid zigzag line in Fig. 7. realized in the ARQ module by a Go-Back-N ARQ with a window length of 1 [14]. 5.2. Simulative Evaluation at the Example of the

UMTS RLC 5. SIMULATIVE EVALUATION AND VALIDATION OF THE FUNCTIONAL MODULES The introduced example of Fig. 7 focuses on the segmentation aspects of the UMTS RLC in the The parameterizable functional modules are implemented Unacknowledged Mode (UM). The UM of the RLC has in the Speciﬁcation and Description Language (SDL), and the responsibility to concatenate SDUs to a PDU of a evaluated with the help of a Modular Object-oriented predeﬁned length, here 128 Byte. The simulative results with and without concatenation are given by the markers. ACKNOWLEDGMENT channel utilization, packet segmentation size of channel = 128 Byte

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